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SP7606ER-L Datasheet(PDF) 5 Page - Exar Corporation |
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SP7606ER-L Datasheet(HTML) 5 Page - Exar Corporation |
5 / 17 page SP7606 12/17/07 rev: A SP7600 ©Copyright 2007 Exar Corporation EXAR RESERVES THE RIGHT TO MAKE CHANGES TO THIS DATASHEET. CALL FOR UPDATES: 1-510-668-7000 page 5 of 17 CIRCUIT DESCRIPTION THEORY OF OPERATION The SP7606 converter is a voltage mode boost controller. The control loop has built in Type 2 compensation coupled with high switching frequency allows the user to use small components when designing the output filter. The equations below show generic relationships as applicable to the boost regulator running in discontinuous conduction mode (DCM) and continuous conduction mode (CCM) of operation. Duty Cycle in continuous conduction mode (CCM) Where Vd= Forward voltage drop of D1 Duty Cycle in discontinuous conduction mode (DCM) Where fsw is the switching frequency L is the inductor Re is the effective resistance of the small signal model Re can be found by using as follows: Setting the output voltage The output voltage of the SP7606 can be set by using an output voltage divider. The internal reference of this part is set to 0.8V. Due to the internal compensation, resistor R1 might need to be chosen according to the desired gain of the compensation loop. This resistor is typically between 100K and 1M ohm. Resistor R2 can be determined by: Vfb=800mV Feedback Voltage R1=Top Voltage divider resistor R2=Bottom Voltage divide resistor 200=the typical impedance of the DD FET For typical applications resistor R1 should be connected between Vout and the DD pin. The DD pin serves as a disconnect for the output voltage divider when the SP7606 is disabled. This feature allows the user to save power when the converter is not running. The typical Impedance of the DD FET when enabled is 200 ohms. If the DD pin is not used the resistor R1 can be connected directly to the Vfb pin to get the proper output voltage. If this type of connection is used the Rdson of the DD Fet can be ignored and the equation becomes: Vfb=800mV Feedback Voltage R1=Top Voltage divider resistor R2=Bottom Voltage divide resistor If the DD pin is not used the converter can be used to boost voltages beyond 38V. A more detailed discussion on this topic can be found in the section “High Voltages Operation”. A 10 to 22pf decoupling capacitor from the feedback pin to ground is recommended when the SP7606 is used with resistor values above 20K in the voltage divider circuit. |
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